1. Field of the Invention
The present invention relates to torque control systems for motor vehicles in general, and more particularly to a torque control system for a drivetrain of all-wheel drive motor vehicles.
2. Description of the Prior Art
Many modern motor vehicles employ all-wheel drive (AWD) systems in which engine torque could be delivered to both front and rear wheels of the vehicle. These systems have been marketed in a number of forms, such as constant AWD systems, part-time AWD systems and on-demand AWD systems.
Some AWD systems have a transfer case, which is controlled by the operator to select two wheel or four-wheel drive. If the operator selects the four-wheel drive condition, the vehicle drives all four wheels continuously. Some of these systems have employed overrunning clutches at two of the wheel to alleviate some of the disadvantages of AWD which result from tire pressure differential and cornering to name a few.
Other AWD systems work automatically and do not require a vehicle operator to intentionally select this condition or to actively engage the system, and they do not have a low range. AWD systems automatically split engine torque between the front and rear wheels as needed. These systems often employ a viscous clutch in the center differential to transfer torque to the drive wheels that are not sensed as slipping. In tight cornering situations and during towing, these AWD systems present a disadvantage. In cornering situations, noise and vibration can result from the AWD system being engaged. While this is not detrimental to the powertrain during short durations, it can be disconcerting to the operator. The AWD system is generally an on-road system and is not designed for off-road use.
Typically, the AWD motor vehicles are provided with a central (or inter-axle) differential. In order to widely control the distribution of the torque, it is necessary that the central differential is arranged to unequally distribute the torque to the front wheels and the rear wheels. There are two methods in the unequal distributing system. One of the methods is a front-weighted distribution which is weighted to the front wheels, and the other is a rear-weighted distribution which is weighted to the rear wheels. In the front-wheel weighted system, larger torque is transmitted to the front wheels. Accordingly, in the front-weighted system, although the vehicle has good straight-ahead stability, it is inferior in steerability because of drifting out of a course. In the rear-wheel weighted system, larger torque is transmitted to the rear wheels. The vehicle has good steerability and driveability on a dry road. However, the vehicle is liable to spin on a slippery road. On the other hand, a disadvantage of the all-wheel drive vehicle is that all the four wheels may slip (slipping spin) at the same time, causing difficulty in driving.
However, in the conventional systems, the standard torque distribution ratio determined by the ratio of the pitch circles can not be changed, unless the diameters of differential gears change. However, it is difficult to change the diameters because of a limitation of space. Since the distribution of torque to the rear wheels can not be set to a large value, the control range of the torque distribution is small. In addition, if the vehicle is driven on a road surface having a low coefficient of friction under slipping conditions and the speed difference is small, the torque distribution can not be controlled. Further, in the conventional systems for the fixed torque distribution control, the control range of the driving conditions is limited to a particular condition such as deceleration. Therefore, it is desirable to control the distribution at a fixed ratio in a wide range of driving conditions including acceleration and steering operation.
Thus, known drivetrains of the all-wheel drive motor vehicles, including but not limited to those discussed above, although proven to be acceptable for various vehicular driveline applications, are nevertheless susceptible to improvements that may enhance their performance and cost. With this in mind, a need exists to develop an improved drivetrain of an all-wheel drive motor vehicle that advances the art.